Electric vehicles (EV) and hybrid vehicles (HV) generate small noise due to theirs structures and it is difficult for a pedestrian to notice approach of these vehicles. In recent years, it becomes popular to equip EV and HV with a vehicle approach warning apparatus, which generates a vehicle approach warning sound such as pseudo engine sound to increase reconcilability when the pedestrian or the like is preset near the vehicle (see Patent Literature 1 for example).
In connection with a vehicle approach warning apparatus, the inventors of the present application have found the following.
In the above-described vehicle approach warning apparatus, if a speaker has a certain abnormality and cannot emit the sound, a vehicle function to warn a pedestrian about approach of the vehicle is lost. When the driver continues to drive without being aware that the vehicle approach warning apparatus does not actually emit the sound, the pedestrian has a difficulty in noticing the approach of the vehicle although the driver is driving on assumption that the vehicle approach warning sound is being emitted. For this reason, it is necessary to detect an abnormality of the vehicle approach warning apparatus.
A conceivable abnormality detection method of a vehicle approach warning apparatus uses a current flowing to a speaker in the sound emission (referred to hereinafter as a speaker current) or a voltage applied to the speaker in the sound emission (referred to hereinafter as a speaker voltage). Specifically, the pseudo engine sound or the pseudo motor sound generated by the vehicle approach warning apparatus has a time-variable frequency component or sound volume, and thus, the speaker current or speaker voltage constantly varies. Therefore, in extreme events such as no-flow of the current in a speaker output wire in open-circuit (disconnection) of the speaker and flow of an excessively large direct current in the speaker output wire in short-circuit of the speaker, it is possible to detect these abnormalities by monitoring a sensor current or a sensor voltage in the sound emission with a current sensor circuit or a voltage sensor circuit and comparing it with a determination threshold for speaker open-circuit abnormality or speaker short-circuit abnormality.
However, in cases of an abnormal mode of freezing and fixing of a speaker cone and an medium abnormal mode, the above-described method cannot detect abnormalities. The medium abnormal mode includes a mode in which the speaker is on the verge of open-circuit, that is, an impedance in a path of the speaker output wire is excessively large. The medium abnormal mode includes another mode such as a layer-short-circuit, in which the speaker is on the verge of short-circuit, that is, the impedance reduces than in a normal state due to partial-short-circuit. Specifically, in these abnormal modes, the speaker current varies in the sound emission, and thus, it is impossible to accurately detect abnormalities by monitoring the sensor current or the sensor voltage in the sound emission with the current sensor circuit or the voltage sensor circuit and comparing it with the determination threshold for speaker open-circuit abnormality or short-circuit abnormality.